Download application specific power supplies

APPLICATION SPECIFIC POWER SUPPLIES
CLASS 'D' AUDIO POWER AMPLIFIERS
Power requirements vary considerably dependent on the application hence the need for application
specific power supplies to optimise the system performance.
Under-designed power supplies result in the amplifier not meeting performance specifications.
Over-designed power supplies increase product cost.
1. POWER AMPLIFIER DESIGN CONSIDERATIONS
The choice of power supply for any given application depends on a number of factors including
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input voltage range (global input)
line voltage regulation
output voltage(s)
voltage tolerance
output voltage regulation
ripple and noise
number of outputs
average and peak output power
peak output current
efficiency
standby power consumption
size and weight
EMI and regulatory requirements
safety and other regulatory requirements
cost
1.1 POWER SUPPLY OUTPUT VOLTAGE AND CURRENT
Factors that affect the required power supply output voltage include
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amplifier output power; rated amplifier power required at minimum supply output voltage
load (speaker) impedance
amplifier output configuration; single-ended or BTL (bridge-tied load) output
amplifier maximum achievable duty cycle
parasitic output path resistance
under-voltage lockout; to avoid poor performance at too low an input voltage
output voltage 'pumping'
BUS PUMPING
In Half-Bridge (single ended) Class D switching stages energy transfer between the power supply output
and the load (speaker) is bi-directional resulting in an interval of energy that was stored in the amplifier's
low pass filter inductor being returned to the power supply. The power supply voltage buses of halfbridge circuits exhibit voltage fluctuations that exceed the nominal values resulting in the terminology
'bus pumping'.
Bus voltage fluctuation creates distortion due to the gain of a Class D amplifier stage being directly
proportional to the bus voltage. Large decoupling capacitors at the amplifier's DC voltage input limit the
transient dV/dt.
Bus pumping does not occur in Full-Bridge circuits because inductor current flowing into one of the halfbridges flows out of the other creating a local current loop that minimally disturbs the power supply
(energy returned back from one side is used in the other side and not returned to the power supply).
EET423 POWER ELECTRONICS
Class D power supply
1
Prof R T Kennedy
2008-2009
AMPLIFIER OUTPUT POWER
The maximum power that an amplifier can deliver to the load without clipping:
HALF-BRIDGE
FULL BRIDGE
PHB , av

PFB , av

( Dsw, max  VCC ) 2  Rload
8 RT 2
( Dsw, max  VCC ) 2  Rload
2 RT 2

PFB , av
4
 4 PHB , av
RT : sum of all of the DC resistances in series with the load: RT = Rload + Rdson + Rind + Rpcb + Rps,out
Rload : loudspeaker resistance:
Rdson : mosfet on -state resistance: HB Rdson
FB 2 Rdson
Rind : filter inductor DC resistance
Rpcb : board traces, connectors, and wires
Rps,out : power supply output impedance
(use the component's resistance at its maximum operating temperature).
Dsw,max: amplifier maximum output duty cycle.
(Dsw is also referred to as the modulation index (M); MMAX is the maximum modulation factor)
Vload, pk  pkFB  2  Vload, pk  pkHB
for the same power supply voltages and load impedance.
Amplifier peak output power occurs at the loudspeaker's peak voltage or peak current
PHB , pk
HALF BRIDGE
FULL BRIDGE
PFBpk
 I out 2 Rload
2
 I out Rload


Vout 2 Rload
(2 RT ) 2
Vout 2 Rload
RT 2

Vout 2  Rload
4 RT 2
 2 PHB
 2 PHF
Vout : amplifier output voltage (loudspeaker voltage)
Iout : amplifier output current
PEAK versus AVERAGE OUTPUT POWER
Regulatory agencies (e.g. U.S. Federal Trade Commission:(FTC) require manufacturers of audio products
to specify the average output power rather than the peak power for consumer audio products.
2
Pav,sine 
2
V pk
Ppk
Vrms


R
2R
2
undistorted sinewave
Amplifier manufacturers sometimes advertise a higher output power at a particular THD level.
Pav (THD  10%)  1.28 Pav (THD  1%)
EET423 POWER ELECTRONICS
Class D power supply
2
Prof R T Kennedy
2008-2009
POWER SUPPLY OUTPUT POWER REQUIREMENTS
Power supply design depends on the target market. Amplifiers for professional recording studios or
laboratory applications may need to provide full output power on a continuous basis; an expensive
requirement only to be used when needed!
Consumers tend to operate amplifiers around 40% continuous power rating; with the outputs only
approaching full power for short periods of time during music peaks. The difference between maximum
available power and typical usage is the basis for the power rating requirements of regulatory agencies.
Regulatory agencies specify a pre-conditioning warm-up period (0  1 hr) at 1/8 the continuous power
rating followed by a short period of time (5  10 min) at the continuous rated power output as a
sufficient test of an amplifier's capability resulting in less expensive amplifier and power supply designs.
1/8 power is regarded as a fair indicator of the average music content of a typical CD however;
rock and heavy metal CDs range around 20% continuous power rating
classical CDs vary widely from very little power up to higher levels than the rock CDs
The full-power operational time depends on the amplifier and its power supply heat dissipation
capability. Higher efficiency amplifiers and power supplies and good thermal management are essential
for long periods at continuous rated output power.
note:
(i)
Each
channel
is
tested
individually,
while
all
other
channels
run
at
1/8
power.
(ii) All channels in the same frequency range tested at full power
POWER SUPPLY OUTPUT VOLTAGE
The required power supply voltage is determined at the lower limit of the power supply's output voltage
tolerance based on the amplifier's rated output power with unclipped output voltage.
HALF BRIDGE
FULL BRIDGE
VCCHB , min

VCCHB , nom

VCCFB, min

VCCFB, nom

EET423 POWER ELECTRONICS
Class D power supply
8PHB ( RT ) 2
( Dsw max ) 2 Rload

2 RT
Dsw max
2 PHB
Rload

RT
2 PFB
Rload
VCCHB , min
1  tolerance
2 PFB ( RT ) 2
( Dsw max ) 2 Rload
Dsw max
VCCFB, min
1  tolerance
3
Prof R T Kennedy
2008-2009
EFFICIENCY
PPSout
Pout max ( amp)

 max ( amp)
High efficiency requirements come at a cost; high efficiency switching power supplies use low onresistance mosfets and synchronous rectification.
Consumer products tend to keep the power supply on, even when the product is supposedly 'off ' in order
to respond to remote controls and use low-voltage front panel power controls. 'Standby' and 'off ' state
power consumption accounts for a large proportion of the consumption of natural resources and energy
and have a number of other important environmental impacts Maximum levels (2 W  0.5 W) are now
an important regulatory issue and are covered by the Energy-using Products (EuPs) Directive.
CURRENT LIMIT
A current limit is set to avoid the power supply limiting the output current during normal operation.
A current limit threshold lower than the maximum required output current results in increased distortion
due to a clipped amplifier output.
I limit ,min 
Dsw,max VCC ,min
RT
REGULATED versus UNREGULATED POWER SUPPLY
Once the power supply voltage and current requirements have been calculated the designer needs to
decide whether to use a regulated or unregulated power supply.
Power supply output voltage ripple can produce amplifier output distortion and audible hum; especially if
the amplifier is an open loop (no output feedback) output stage design. The amplifier design therefore
affects the power supply type with regulated supplies recommended for open-loop amplifiers.
UNREGULATED POWER SUPPLIES
Unregulated power supplies can be less expensive for amplifiers with low power requirements but the
cost (and the size and weight) of the transformer will increase as power requirements go up
Unregulated power supply line voltage and output load changes produce ±20% (or more) output voltage
variations resulting in the VPSout,max  1.5 VPSout,min. Higher cost higher voltage components are required
thereby reducing some of the savings gained by not using a regulated power supply.
REGULATED POWER SUPPLIES
LINEAR REGULATORS
Linear regulators are NOT recommended for input line regulation due to their low efficiency and higher
cost of line transformers and thermal management.
Linear regulators are used as post regulators for auxiliary low-voltage outputs (±12V, 5V, 3.3V ).
EET423 POWER ELECTRONICS
Class D power supply
4
Prof R T Kennedy
2008-2009
SWITCHING REGULATORS
Line and load regulation of a switching power supply depends on the type of feedback
Secondary-side feedback (regulator + opto-isolator) provides the best output regulation as it senses the
power supply output voltage(s) directly and provides feedback to a primary side regulator.
Primary-side feedback using a transformer primary side winding to regulate the output voltage, rather
than sensing the output voltage directly, is a lower cost poorer performance approach.
Power supply design should take care to ensure that variations in the amplifier's power supply voltage
due to load variations on the auxiliary outputs do not create audible amplifier noise.
POWER SUPPLY COMPONENT REQUIREMENTS
Switching power supply magnetic components (transformers and inductors) require a peak load current
capability to avoid saturation.
Low ESR power supply output capacitors are required to minimize losses due to the high-frequency
ripple current.
Low capacitor ESR also provides a lower source impedance (looking back from the amplifier) and affects
the amplifier's overall damping and efficiency.
OUTPUT PROTECTION
Regulatory agency's safety requirements require output protection to prevent damage, overheating and
fire due to short circuits, internal component failures or other abnormal conditions.
Switching power supplies usually have inherent current limiting and current shut down pwm controllers
whereas unregulated power supplies tend to rely on fuses or circuit breakers.
EET423 POWER ELECTRONICS
Class D power supply
5
Prof R T Kennedy
2008-2009
DESIGN EXAMPLE: SINGLE-CHANNEL FULL- BRIDGE AMPLIFIER
PFB,max
amplifier maximum output power
20 W
ηmax
amplifier maximum efficiency
90%
Dswmax
amplifier maximum duty cycle
0.8
Rload
amplifier loudspeaker impedance
8Ω
RT
total output resistance
8.2 Ω
power supply output voltage tolerance
± 10 %
2 PFB ( RT ) 2
( Dsw max ) 2 Rload
2 PFB
Rload

VCC min

8.2 2  20
0.8
8

22.919 V
VCCnom

VCC min
1  tolerance

22.919
 25.466 V
1  0.1
Dsw max VCCnom
RT

0.8  25.466
8.2
I limit min

PPSout

Pout max (amp )
 max ( amp )
EET423 POWER ELECTRONICS
Class D power supply


RT
VCC min
( Dsw max
 2.485 A
20
 22.22 W
0.9
6
Prof R T Kennedy
2008-2009
load voltage, load current, peak power (instantaneous power), power.
All the power delivered to the load comes from the power supply and from the decoupling capacitors.
Note that the frequency of the instantaneous power delivered to the load is twice the frequency of the
audio input signal. The frequency of power supply output current is also twice that of the audio signal .
Instantaneous Power and PSU current
EET423 POWER ELECTRONICS
Class D power supply
7
Prof R T Kennedy
2008-2009
Single-Ended Outputs
Many designs use amplifiers with single-ended outputs because they only require half as many transistors
as a full-bridge output, and integrated amplifiers with single-ended outputs only require one output pin
instead of two.
Single-ended amplifiers also have a few disadvantages compared to amplifiers with bridge-tied load
(BTL) outputs. Single-ended amplifiers require either split positive and negative power supplies or DC
blocking capacitors. If DC blocking caps are used they need to be large in order to prevent them from
affecting the low-frequency performance of the amplifier.
DC blocking caps can also cause audible pops as they charge up to Vcc/2 when the amplifier is turned on.
A resistor divider from Vcc to ground can be used to charge the capacitor up to Vcc/2 at a relatively slow
rate when the power is turned on, minimizing or eliminating the pop.
BTL Outputs
Amplifiers with BTL outputs are popular because they do not require DC blocking caps even when
operating with a single positive power supply. DC blocking caps limit the low-frequency response of the
amplifier and can be quite large.
BTL amplifiers have another advantage over amplifiers with single-ended outputs - the maximum peakto-peak voltage that a amplifier with BTL outputs can apply to the speaker is twice the power supply
voltage, which in turn means that up to four times as much output power can be delivered to the load
compared to a single-ended amplifier.
This can be a big advantage in applications where the power supply voltage is limited, especially in
portable applications where the amplifier is operating off of a battery.
.
EET423 POWER ELECTRONICS
Class D power supply
8
Prof R T Kennedy
2008-2009